1. Field of the Invention
The present invention relates to calibration of a transmitter to remove distortions, and more particularly to on-signal quadrature modulator calibration using an output signal detector to remove DC, phase and amplitude imbalances.
2. Description of the Related Art
Many modern communications systems combine in-phase (I) and quadrature phase (Q) components in a transmitted signal. The I and Q components can be generated and combined in the digital domain without distortion. The combined digital signal must then be converted to an analog signal prior to up-conversion and transmission, which generates an undesired image that is difficult and expensive to eliminate. Alternatively, the I and Q components may be separately converted to analog signals, which are then combined in the analog domain. The analog processing components are not perfectly matched, so that several types of distortion are introduced into the signal path. These distortions include DC offsets and amplitude and phase imbalances. For example, after conversion to analog, an I DC offset is added to the I signal channel and a separate Q DC offset is added to the Q signal channel. One or more mixer stages are used to up convert each of the analog signals to a radio frequency (RF) level suitable for transmission. The local oscillator (LO) splitter carrier signals and the mixer stages are not perfectly matched resulting in amplitude and phase imbalances that cause distortions in the transmitted signal.
The distortions, caused by DC, amplitude and phase imbalances, degrade the performance of digital communication systems including systems employing digital signal processing (DSP) techniques. Amplitude and phase imbalances cause variations in constellation plots that degrade communication effectiveness. Large gain or phase imbalances can effectively disable reliable communications, particularly higher density modulation techniques used to achieve higher data transmission rates.
In many legacy systems, distortions have been measured and calibrated at the time of manufacture. Imbalances, however, can drift over time due to various factors including, for example, temperature and aging. Static compensation at the time of manufacture does not address the dynamic factors that affect radio operation during use. Several dynamic compensation techniques are known. Some methods use off-line signals, such as training signals or the like, which require taking the system off-line for calibration. Such off-line methods degrade overall system efficiency by consuming valuable time to perform calibration and compensation. Vital packets may be missed during the calibration process since the antenna(s) must be disconnected or otherwise disabled to prevent inadvertent transmissions. Some calibration techniques negatively impact the wireless medium, such as undesirable signal splattering. It is desired to provide an automatic and relatively simple imbalance compensation solution that operates on the actual communication signals (e.g., on-signal) without having to go off-line or perform separate calibration cycles.